Canon REBEL T3I EOS 600D User Manual page 145

AE lock and exposure compensation cannot be set.
Changing the shutter speed or aperture during movie shooting is not
recommended since the changes in the exposure will be recorded.
If you use a lens whose maximum aperture changes while you zoom,
you should not zoom while shooting a movie. Changes in the exposure
may be recorded.
If you shoot a movie under fluorescent lighting, the movie image might
flicker.
With ISO Auto, the standard movie exposure will usually be obtained
even if the light level changes.
When shooting a movie of a moving subject, a shutter speed of 1/30 sec.
to 1/125 sec. is recommended. The faster the shutter speed, the less
smooth the subject's movement will look.
Focus and shoot the movie.
6
The procedure is the same as steps 2
and 3 for "Autoexposure Shooting"
(p.142).
k Shooting Movies
145